The invention relates to a method and device having a collimator for limiting a beam of high-energy radiation directed from an essentially point-shaped radiation source onto an object to be treated for stereotactic, conformal radiation therapy of tumors, wherein the collimator has a scanning device with a collimator and a drive mechanism for scanning an area of an object being treated with a beam of rays defined by the collimator.
Collimators for limiting a beam of high-energy radiation are used for diagnostic purposes and for the treatment, in particular, of tumors. Here, the collimators are used to limit the beam, so that healthy tissue lying next to the diagnostic or treatment area is protected as much as possible from the radiation in order to prevent injury or to reduce it to a minimum.
Collimators were originally designed to delimit only the size of an irradiation field. If only X-rays were used for imaging, the patient was not seriously impaired. Only therapeutic irradiation with high-energy rays, e.g. to destroy tumorous tissue, damaged healthy tissue in the excessively irradiated areas, i.e. outside of the ill tissue to be irradiated. These excessively irradiated areas were generated since the contour of the ill tissue was not simulated by the collimators and also since half shadows were generated at the boundaries of the irradiated area, where, in particular with large irradiation fields, the entire strength of the shielding material was not available, since it was not oriented parallel to the rays.
One example of such a collimator of older design is shown in U.S. Pat. No. 2,675,486. This document concerns a collimator for limiting high-energy rays, comprising four ray-delimiting blocks, which can be displaced in one plane using bordering side surfaces, such that a square ray limitation of different sizes can be set. Since tumors tend to have a round rather than square shape, there are large excessively irradiated corner areas. With large irradiation fields, one moreover obtained large half shadow areas, since the block limits no longer extend parallel to the divergent path of rays.
For this reason, the experts tried to solve these problems:
Departing from a collimator of the above-mentioned type, DE 20 53 089 A1 proposes, for the field of X-ray imaging which is related to the field of the inventive object, providing shielding elements in the form of bordering triangles, in order to obtain an approximately circular irradiation field, which corresponds more to the shape of an irradiation area, such that excess irradiation caused by the corners of the above-mentioned square ray limitation is prevented by approximately 30%. The remaining excess irradiation and half shadow formation do not represent a serious problem, since it only concerns X-rays for imaging and not therapeutic irradiation with rays of substantially higher energy.
DE 15 89 432 A1 proposes a collimator to be used with the relevant, ionizing, high-energy rays which are suited for the treatment of tumors, wherein bordering wedge-shaped irradiation shielding elements can be displaced in one plane such that hexagonal, octagonal or rectangular openings can be combined. This collimator, however, does not sufficiently simulate the tumor shape and provides no suppression of half shadows. For large irradiation fields, wherein the path of rays extends at a great inclination to the limitation of the shielding material, a large half shadow is generated.
DE 10 37 035 B is also based on a collimator of the type of the first-mentioned document, wherein the four ray-limiting blocks are divided into two parts along an inclined line for high-energy therapeutic rays, wherein the line extends to that location where the inner and end surfaces (i.e. the surface bordering the next block) meet. One thereby obtains a main and a side part of each block which can be mutually displaced. This permits formation of different contours, which also reduces excessive irradiation compared to square ray limitation. The problem of simulation of the shape of a tumor or another area to be irradiated is, however, only very insufficiently solved, and the problem of half shadows is not solved at all.
DE 15 64 765 A1 finally solves the problem of half shadows. This document is also based on a collimator of the type disclosed in the first-mentioned document, with four bordering radiation-limiting blocks which can be displaced in a plane. It is based on the object to obtain a field with sharp borders, i.e. a field without half shadows. Towards this end, it is proposed to design and pivotably displace the blocks in such a fashion that the front ends forming the radiation limit are directed onto the radiation source in each setting. The material of the blocks thereby always shields the full radiation. However, this collimator only forms square irradiation fields, such that large excessively irradiated areas on the corners must be accepted.
FR 2 524 690 addresses both the problem of excessively irradiated areas, and the half shadow problem. This document proposes to arrange bordering plates, which can be displaced in a plane, in several planes for preventing or reducing half shadows, in order to obtain a stepped, truncated pyramid-shaped ray-limiting opening. In this fashion, the half shadow is minimized. It only appears in that area where the rays cross the stepped shape. The larger the surface to be limited, the larger becomes this stepped area of the half shadow which still remains despite this measure. A further disadvantage of this approach consists in that only polygons can be formed as irradiation field limitation in dependence on the number of plates, and shaping of the true tumor contour is not possible.
EP 1 367 604 A1 discloses a device for concentrating an X-ray into a micro-X-ray, wherein the concentration is obtained by reflection on reflecting inner surfaces of a capillary tube. This capillary tube is formed by displacing concentrically arranged rod segments, which can be displaced and adjusted by screws. This device only permits very limited point irradiation. Moreover, the effect of reflection on reflecting inner surfaces is not suited for therapeutic rays which are in a megavolt range.
In order to improve the simulation of the tumor shapes and reduce the excessive irradiation to a minimum, one finally started to use changeable fixed collimators. The tumor shape was thereby detected from different spatial directions, and several fixed collimators were produced for each irradiation, which were then used for irradiation from the different directions. This is advantageous due to exact shaping and exact adjustability of the limitations to the path of rays, wherein any half shadow is eliminated. The disadvantage is, however, that the method is complicated, requiring permanent collimator change, which consumes a great deal of time on expensive devices, and is also costly since many collimators must be produced for each irradiation, which are useless after that, since they are determined for use for one patient only and can be used for that patient only within a limited time period, since the shape of the tumor permanently changes due to growth, decrease, or shape changes.
In order to reduce this effort, multileaf collimators were generated, having a plurality of narrow, closely adjacent leafs (i.e. diaphragm leaves), with which the shape of a tumor can be simulated via actuation of the leaves. These multileaf collimators were initially advantageous in that almost any shape could be quickly adjusted, but are disadvantageous in that the mechanism with adjustment means for each leaf is very complex and also since a more or less large half shadow was generated on each limit of the irradiation field by a leaf, in dependence on the separation between the leaf and the axis of the path of rays.
In order to avoid such half shadows, EP 1 153 397 B1 proposes leaves having adjustable front edges, wherein a mechanism always adjusts them parallel to the path of rays. This requires, however, an even more complex mechanism of the multileaf collimator.
In order to avoid this complex mechanism and be more flexible in shaping a surface to irradiated, DE 199 22 656 A1 finally proposes a scanning device with a collimator opening which is sufficiently small that the areas of the object to be irradiated can be irradiated with sufficient accuracy (FIG. 3). In the above-mentioned proposal, a small collimator opening provides great accuracy, but slower scanning. A large diaphragm opening provides faster scanning but not the required accuracy. The use of multi-hole plates for generating a bundle of several scanning rays (FIGS. 5 and 5a) thereby did not reduce the irradiation to a satisfactory degree. The multi-hole plate was fixed relative to the irradiation area, and even smaller diaphragm openings had to be used for exact irradiation of the edge areas, i.e. the plates had to be changed.
In order to increase the scanning speed and still obtain high accuracy, DE 101 57 523 C1 finally proposes a collimator with several collimator openings of different sizes, which can optionally be brought into the path of rays. This was preferably effected using a revolver-like mechanism which rotates a round plate having openings of different sizes. A material thickness of 6 to 10 cm is necessary for shielding the high-energy rays which are used in therapy today. In this fashion, one either obtains a very heavy collimator, or one must make do with a few, e.g. three opening sizes. Even with such a limitation, the openings which are not used must be covered to prevent the generation of regions which are only shielded by an insufficient material thickness. A shielding plate is required in addition to the plate with openings, which must also have a thickness of several centimeters. For this reason, the collimator becomes relatively heavy, which correspondingly increases the requirements for guides and drives. This collimator is also disadvantageous in that, for the above-mentioned reasons, only a few of the fixed collimator openings are available, thereby strongly limiting the variability of ray collimation. In particular, for the above-mentioned reasons, it is not possible to provide large openings of different diameters for initially treating an area of the surface to be irradiated, which is as large as possible in order to subsequently treat the edge areas with stepped finer bundles of rays. Since the dwell time of ray application for each point of a surface is several seconds, the scanning of an area with fixed sizes of ray bundles is more time-consuming than with sizes which can be optimally adjusted. This is the case, in particular, when the ray bundles are narrower than possible with regard to the irradiation area. This increases the overall treatment time. This is not only unpleasant for the patient who must remain stationary, but also reduces the number of treatments that can be performed on one device, which is economically very important in view of the high acquisition and operating costs of such devices. Moreover, the accuracy of edge area detection is limited, which is critical in areas such as bordering nerves.
EP 0 382 560 A1 discloses an iris diaphragm as a ray-limiting means, and mentions irradiation by “scanning”. It does not concern a scanning motion of the type mentioned in DE 199 22 656 A1, wherein rays are applied onto a surface through the scanning motion of a limited ray, wherein these applications are sequentially performed from different spatial angles by displacing a gantry with radiation source, ray limitation and scanning device about the patient. In EP 0 382 560 A1, the above-mentioned circling of the area to be irradiated is called “scanning”. The application from a direction is not effected by scanning of an area, i.e. “scanning” as usually understood in technology. The irradiation to be applied in each case from a direction onto a treatment surface is rather approximately adjusted with the iris diaphragm, as shown in FIGS. 2 through 5, and described in the description of EP 0 382 560 A1. These surfaces are then always polygons in accordance with the diaphragm leaves of the iris diaphragm, i.e. approximately circles. This rough definition of an area cannot simulate the tumor shape and therefore destroys the healthy tissue, which is also irradiated. For this reason, the proposal of EP 0 382 560 A1 has disadvantages which can no longer be accepted today, and have already been overcome by the technical development proposed by DE 199 22 656 A1 and DE 10 157 523 C1.
The invention is therefore based on a scanning device as disclosed in DE 101 57 523 C1. This document corresponds to the collimator mentioned above. The invention is based on the problem of configuring a collimator of the type previously described above and method for use thereof, such that a variable opening size of the diaphragm opening can be achieved with a high degree of shielding and low overall height.